This solution outlines how to run a computer discovery against only workstations and not servers.
1. Edit the Computer discovery (or create a new one) and go to the Scopes tab
2. Click Add at the top
3. On the bottom of the new window click Add for "Add scope using a query"
4. On the Find drop down menu choose "Custom Search"
5. Click the Advanced tab
6. By default it will select "Entire Directory" on the top, make sure to choose the target domain (will need to create multiple queries if multiple domains are being targeted)
7. For the LDAP query, type in the below
8. Click "Find Now" to ensure the results are what you are expecting
9. Click OK and then OK on the Select Scopes window. If you need to see what the query was, you can hover the mouse pointer over the added query to see the LDAP query